1. Field of the Invention
This invention relates to flue gas scrubbers. More particularly, this invention relates to flue gas scrubbers employed to remove sulfur dioxide from flue gases generated in utility and industrial steam generating units, incinerators and the like. Still more particularly, this invention relates to a process for controlling scale in flue gas scrubbers in which sulfur dioxide is removed from flue gases by scrubbing with an aqueous solution or slurry.
2. Description of the Prior Art
The use of sulfur-containing fuels, particularly coal, in industrial processes, such as in the generation of steam and the like, produces flue gases that cannot be discharged directly to the atmosphere because of the impact on the environment of their sulfur dioxide contents. Such flue gases, therefore, are usually subjected to any of several treating systems designed to remove, or at least reduce, their sulfur dioxide content. One such system is known as a wet scrubbing system in which the flue gas is treated, or scrubbed, with an aqueous alkaline liquor in the form of a slurry or solution to convert the sulfur dioxide content to an alkaline sulfite or sulfate salt. This salt is then removed as scrubbing liquor and is bled from the system.
When a slurry of lime (calcium oxide) or limestone (calcium carbonate) is used as the scrubbing liquor, the sulfur dioxide of the flue gas is converted to calcium sulfite and/or calcium sulfate salts. These calcium salts not only have a low solubility in the scrubbing liquor but also tend to produce highly tenacious deposits of scale in the flue gas desulfurization system (FGD) particularly on surfaces in the demister and spray section and on the packing and associated ductwork. The scale thus produced reduces system throughput by increasing the pressure drop across the system, with corresponding reduction in the efficiency of the system in removing sulfur dioxide from the flue gas. The labor and down time associated with periodically shutting the system down to manually clean the system of deposited scale is costly as is replacement of components of the system that may be damaged by the scale beyond the point of salvage.
Various procedures have been proposed for controlling calcium scale when using lime and limestone slurries in FGD systems one of which involves the use of various additives to the slurry designed to inhibit the formation of scale. U.S. Pat. No. 4,177,245, for instance, discusses the reduction of calcium scale formation by inhibiting the oxidation of calcium sulfite to the more insoluble calcium sulfate through the use of additives such as magnesium oxide (U.S. Pat. No. 3,919,393), thiosulfate (U.S. Pat. No. 3,937,788) and hydroquinone (U.S. Pat. No. 3,485,581), and further discloses in detail the use of nitric oxide for the same purpose. U.S. Pat. No. 4,216,087 discloses the use of 2-hydroxyethyl iminodiacetate for the same purpose while U.S. Pat. No. 4,284,513 discusses a procedure for inhibiting the formation calcium scale by providing a film on the surfaces of the FGD system through the use of any of various organic phosphor compounds, especially phosphonates, phosphoric acid esters and polyol phosphoric acid esters.